8 ms·

If these sensors have a common failure mode (eg. always showing lowest possible value after blowing up), this could be bad. If they just produce junk, it's unli

If these sensors have a common failure mode (eg. always showing lowest possible value after blowing up), this could be bad. If they just produce junk, it's unlikely they would agree, and MCAS would be turned off.

cjbprime 7y ago
The common failure mode would be e.g. freezing, perhaps in identical positions, while on the ground before takeoff. You can't easily detect it before takeoff because the sensor requires airflow.
- blattimwind 7y ago
  I assume you can walk up to it and turn it by hand to at least see that the mechanical part is moving freely as-of now.
  - bgdam 7y ago
    Can't always test the mechanics like that. Ex: Pitot tubes are just hollow tubes. The interiors of pitot tubes getting iced over has crashed planes. This wouldn't even be visible from the outside, and there is no test you could do.
    - maxmunzel 7y ago
      Couldn’t you just blow in?
      - chipsa 7y ago
        No. The tube leads to a pressure sensor. There's generally no actual airflow moving through the tube in use.
  - kayfox 7y ago
    It can freeze from icing or other low temperature conditions after takeoff. The sensors are heated to prevent this, with a system in place to detect if the heater has failed, but they can still jam from a heater failure that is not detected or heavy icing.
    - magicalhippo 7y ago
      Surely this could be easily detected, even with too low airspeed to give reliable readings the sensors should flap around slightly and thus provide noise. If they're frozen, there should be very little noise in comparison. In addition you should be able to cross-correlate with gyro and airspeed sensor, not to check the absolute AoA, but certainly determine if delta-AoA is near zero (frozen) or not.
      - chipsa 7y ago
        The sensors should be on good bearings, in order to promote them moving freely. Them flapping around slightly won't provide much noise. Which is probably drowned out by all the other noise on an aircraft.
        magicalhippo 7y ago
        > Them flapping around slightly won't provide much noise. Which is probably drowned out by all the other noise on an aircraft. But the readings with the vane flapping around should have a significantly higher variance than the readings where the vane has frozen stuck. In addition, a sliding average should show the mean moving around significantly compared to the frozen readings. Surely they should be able to discriminate between these cases based on those factors, especially when cross-correlated with other sensors.
tzs 7y ago
Do any planes do cross-sensor sanity checks? For example, if the AoA sensors say the nose is too high and the flight control software trims down to compensate and then it doesn't see the AoA sensors report lower pitch, couldn't it check the Attitude Indicator (AI)? Angle of attack and attitude aren't necessarily the same, because the wind vector is not necessarily parallel to the ground, but AoA changes and AI changes should be well correlated most of the time. If trim changes are showing up in the AI data but not the AoA data, then you've probably got malfunctioning AoA sensors.
- jsmith45 7y ago
  My understanding is that attitude is only a reasonable approximation of Angle of attack (assuming winds are parallel to the ground) if you are in level flight. My understanding is that with still air, AOA is basically the difference between the attitude and the vertical direction of travel. (i.e. in still air you are climbing at an angle of 5 degrees with the nose pointed up at 10 degrees, your AOA would be 5). So in my possibly flawed understanding the better way to approximate real angle of attack when the wind is parallel to the ground would probably be to take airspeed and rate of climb data, perform trig to get the angle of climb or descent as seen from the air reference frame (cf. airspeed vs groundspeed). Then subtract that from the gyro indicated attitude, and you get an angle of attack approximation that assumes no up or downdrafts. That combination might yield something better to compare against the AOA sensors than just the attitude value. (Although I would agree that if it changing the trim causes the attitude to change but not the AOA sensor output, something is probably wrong.)

cjbprime 7y ago

The common failure mode would be e.g. freezing, perhaps in identical positions, while on the ground before takeoff. You can't easily detect it before takeoff because the sensor requires airflow.

blattimwind 7y ago
I assume you can walk up to it and turn it by hand to at least see that the mechanical part is moving freely as-of now.
- bgdam 7y ago
  Can't always test the mechanics like that. Ex: Pitot tubes are just hollow tubes. The interiors of pitot tubes getting iced over has crashed planes. This wouldn't even be visible from the outside, and there is no test you could do.
  - maxmunzel 7y ago
    Couldn’t you just blow in?
    - chipsa 7y ago
      No. The tube leads to a pressure sensor. There's generally no actual airflow moving through the tube in use.
- kayfox 7y ago
  It can freeze from icing or other low temperature conditions after takeoff. The sensors are heated to prevent this, with a system in place to detect if the heater has failed, but they can still jam from a heater failure that is not detected or heavy icing.
  - magicalhippo 7y ago
    Surely this could be easily detected, even with too low airspeed to give reliable readings the sensors should flap around slightly and thus provide noise. If they're frozen, there should be very little noise in comparison. In addition you should be able to cross-correlate with gyro and airspeed sensor, not to check the absolute AoA, but certainly determine if delta-AoA is near zero (frozen) or not.
    - chipsa 7y ago
      The sensors should be on good bearings, in order to promote them moving freely. Them flapping around slightly won't provide much noise. Which is probably drowned out by all the other noise on an aircraft.
      - magicalhippo 7y ago
        > Them flapping around slightly won't provide much noise. Which is probably drowned out by all the other noise on an aircraft. But the readings with the vane flapping around should have a significantly higher variance than the readings where the vane has frozen stuck. In addition, a sliding average should show the mean moving around significantly compared to the frozen readings. Surely they should be able to discriminate between these cases based on those factors, especially when cross-correlated with other sensors.

tzs 7y ago

Do any planes do cross-sensor sanity checks? For example, if the AoA sensors say the nose is too high and the flight control software trims down to compensate and then it doesn't see the AoA sensors report lower pitch, couldn't it check the Attitude Indicator (AI)? Angle of attack and attitude aren't necessarily the same, because the wind vector is not necessarily parallel to the ground, but AoA changes and AI changes should be well correlated most of the time. If trim changes are showing up in the AI data but not the AoA data, then you've probably got malfunctioning AoA sensors.

jsmith45 7y ago
My understanding is that attitude is only a reasonable approximation of Angle of attack (assuming winds are parallel to the ground) if you are in level flight. My understanding is that with still air, AOA is basically the difference between the attitude and the vertical direction of travel. (i.e. in still air you are climbing at an angle of 5 degrees with the nose pointed up at 10 degrees, your AOA would be 5). So in my possibly flawed understanding the better way to approximate real angle of attack when the wind is parallel to the ground would probably be to take airspeed and rate of climb data, perform trig to get the angle of climb or descent as seen from the air reference frame (cf. airspeed vs groundspeed). Then subtract that from the gyro indicated attitude, and you get an angle of attack approximation that assumes no up or downdrafts. That combination might yield something better to compare against the AOA sensors than just the attitude value. (Although I would agree that if it changing the trim causes the attitude to change but not the AOA sensor output, something is probably wrong.)